1. Field of the Invention
The present invention relates to a rotary electric actuator that functions as a torque motor. The rotor of the actuator is deflected in discrete increments in accordance with the current level applied to a coil.
2. Description of Prior Developments
U.S. Pat. No. 3,370,198 discloses a stepper motor wherein a permanent magnet rotor is rotated through a distance of one hundred eighty degrees in response to the application of a current pulse to a field coil. When the current pulse is removed, the rotor rotates through an additional one hundred eighty degree distance so as to assume its starting position.
The additional one hundred eighty degree motion is made possible by a second permanent magnet mounted on the stator frame so as to generate a magnetic flux moving in a reverse direction to the flux generated by the field coil. When the current pulse is removed from the coil, the magnetic flux generated by the second permanent magnet interacts with the poles of the rotor magnet to rotate the rotor an additional one hundred eighty degrees.
U.S Pat. No. 4,447,793 discloses a two-position rotary actuator that includes a rotor comprising a permanent magnet encircling a rotary shift. An electrical stator includes an annular magnetic housing and three radial pole pieces extending toward the rotor axis. An electrical winding encircles each pole piece. The windings are energizable so that current flow in one direction causes the permanent magnet to be aligned with one pole piece and a gap formed between the other two pole pieces. Current flow in the reverse direction causes the permanent magnet to be aligned with a second pole piece and a gap formed between the remaining pole pieces.
U.S. Pat. No. 5,097,162 discloses a stepper motor that includes a permanent magnet rotor and an annular stator encircling the rotor. The stator includes two semi-cylindrical pole pieces and a field coil concentric with the rotor axis.
U.S. Pat. No. 4,976,237 discloses an engine air intake valve operated by a rotary electrical actuator. An electrically conductive wire is wound around an annular cylindrical stator to define two diametrically-opposed magnetic poles. A rotor disposed within the cylindrical stator carries two permanent magnets that are polarized in a plane passing through the rotor axis.
When the electrical winding on the stator is energized by a D.C. voltage, the rotor is deflected in accordance with the magnitude of the applied voltage and current flow through the winding. The rotary electrical actuator is thus enabled to control the position of the associated air intake valve so as to obtain a range of different air flows through the valve.
U.S. patent application, Ser. No. 882,787, filed May 14, 1992, discloses a variation of the above-described electrical actuator wherein the magnetic poles on the stator ring are formed by rod-shaped pole pieces extending radially from the stator ring. The pole pieces include electrical windings which, when energized, establish two diametrically-opposed magnetic poles on the stator ring. The rod-shaped pole pieces and electrical windings can be low cost, commercially available solenoid coil assemblies.
In the above-described electrical actuator constructions shown in U.S. Pat. No. 4,976,237 and copending patent application, Ser. No. 882,787, the rotor deflection exhibits essentially a straight line or linear relationship with the applied current. Slight variations in the applied current thus produce corresponding variations in the rotor position.
There are some situations where it is desired that the rotor move stepwise to a predetermined deflected position and remain in that position even though the value of the applied current fluctuates slightly. The present invention is concerned with an electrical actuator having such a characteristic.